Halftone dot-growth technique based on morphological filtering

ABSTRACT

A method for converting an original halftone bitmap image to a color converted halftone bitmap image by morphological filtering of the original image to compensate for tone and color reproduction characteristics comprising: providing an original halftone bitmap image. The method includes providing a set of asymmetrical morphological filters that have the property that when they are applied recursively in order, the features in the halftone bitmap image are dilated or eroded by varying amounts by extending or reducing the edges first in one direction, then in two directions, etc. The method further includes segmenting the original halftone bitmap image into blocks and for each block: applying the set of morphological filters to the original halftone bitmap image to produce a set of modified halftone bitmap images; estimating the percent dot area of the original halftone bitmap image and the set of modified halftone bitmap images; applying a predetermined dot-gain to the percent dot area of the original halftone bitmap image to produce a modified percent dot area; selecting the modified halftone bitmap image whose percent dot area is closest to the modified percent dot area to produce a block of the color corrected halftone bitmap image; replacing the original halftone bitmap image with the combined blocks of selected modified halftone bitmap image.

FIELD OF THE INVENTION

[0001] This present invention relates in general to digital halftoningimaging systems and more particularly to a digital halftone dot-growthtechnique based on morphological filtering.

BACKGROUND OF THE INVENTION

[0002] Digital halftoning refers to any algorithmic process that createsthe illusion of continuous-tone images from the judicious arrangement ofbinary picture elements (pixels).

[0003] RIPing (Raster Image Processing) is the digital image processingtechnique that takes a continuous-tone image and original graphicalelements and provides a binary halftoned output image file. RIPing animage file usually can include the steps of spatial filtering,interpolation, tone and color modification, and digital halftoning.

[0004] There is a need to control the size of the dots in a pre-existinghalftone bitmap. This bitmap could have been generated by a digitalscreening system (i.e., a RIP) or it could have been generated byscanning an optical piece of film to create a digital bitmap file. Ingeneral, these bitmap files were created with some output device inmind. As such, the dot pattern was created to produce a given density todot percentage relationship tailored to the target output device. Ifthis bitmap file were printed on an output device that had differentdot-gain characteristics than the target device the density as afunction of dot-area percentage will be different. Thus, in order toachieve the desired dot percentage to output density relationship of thetarget device, on a different device, the bitmap file for the targetdevice must be modified according to the dot-gain differences betweenthe target device and the given output device.

[0005] For most traditional digital-proofing scenarios this dot-gaincompensation is done by re-RIPing the original art with a modifieddot-gain curve. This modified dot-gain curve accounts for the dot-gaindifferences between the target device and the proofing system. Thus, acustom bitmap file needs to be generated for each output device. Themain drawback to this paradigm is that the proofs may be made on systemsthat do not use the same screening technology as was used to create thetarget bitmap file. Thus, in order to support a RIP-once-output-many(ROOM) workflow the dot-gain compensation needs to be applied to thebitmap file directly.

[0006] Bitmap files can consist of single channel bitmaps such as thosefor a single color device (e.g., a black-and-white laser printer), thosefor a four-color (e.g., CMYK) output device (e.g., a printing press or agraphic-arts proofer), or any output device that utilizes bitmap filesof any number of channels (e.g., a multi-ink inkjet printer).

[0007] Several approaches for dot-gain modification of bitmaps have beenproposed. The dot gain may be adjusted for each of the primary colorscyan, magenta, yellow, and black. A description for how to do this isdisclosed by Spence in U.S. Pat. Nos. 5,255,085 and 5,293,539 titled“ADAPTIVE TECHNIQUE FOR PROVIDING ACCURATE TONE REPRODUCTION CONTROL INAN IMAGING SYSTEM” and “METHOD AND APPARATUS FOR CALIBRATING TONEREPRODUCTION IN A PROOFING SYSTEM”. Here, percent dot area is calculatedusing the Murray and Davies equation from measured densities.

[0008] Denber, et. al. disclose a method of shifting and ANDING a bitmapimage with itself to thin the image displayed in U.S. Pat. No. 5,250,934“METHOD AND APPARATUS FOR THINNING PRINTED IMAGES” and also teaches amethod of setting a bit to an intermediate level if it is diagonallybetween two active bits using shifting, logical AND, and a logical ORoperation.

[0009] Mailloux, et al. discloses using a 4×4 input to a lookup table todetermine how to operate on the central 2×2 pixels to implement halfbitor fullbit dilation and erosion in U.S. Pat. No. 5,483,351 titled“DILATION OF IMAGES WITHOUT RESOLUTION CONVERSION TO COMPENSATE FORPRINTER CHARACTERISTICS”. This requires knowing some of the surroundingpixels when deciding how to dilate or erode the pixels in the center.

[0010] Eschbach teaches in U.S. Pat. No. 5,258,854 titled “CONVERTINGBETWEEN WRITE-WHITE, WRITE-BLACK, AND NEUTRAL BITMAPS”, how to resizebitmap images in small amounts less than one full bit in size. Eschbachstates that the erosion and dilation may be by differing amounts in thex and y directions, and that the amount of resizing may be a fraction ofa full pixel.

[0011] Loce, et al. teaches logically combining two morphological filterpairs and an original image to create an output image in U.S. Pat. No.5,680,485 titled “METHOD AND APPARATUS EMPLOYING EROSION-BASED FILTERPAIRS FOR IMAGE MAPPING”. The morphological filters described areerosion filters, one of which has less erosion than desired and theother having more erosion than desired. Logically combining the originalimage with two eroded images provides for a method of obtaining anintermediate result.

[0012] Eschbach describes a method of resizing an input bitmap in U.S.Pat. No. 5,208,871 titled “PIXEL QUANTIZATION WITH ADAPTIVE ERRORDIFFUSION”. Eschbach simulates a scan of an output image from an inputbitmap such that the scan resolution is different from the input bitmap.Error diffusion is utilized to quantize the output bitmap into thedesired output bit resolution. This example uses error diffusion tospread out the error in the quantization of a multilevel pixel into areduced number of output states.

[0013] U.S. Pat. No. 6,115,140, title “Method and System for Half ToneColor Conversion”, inventors Bresler and Nosko, issued Sep. 5, 2000teaches using a descreened version of an original image, dilated anderoded versions of the original image to select a combination of theoriginal, dilated, and eroded image to effect a dot gain or tone scalechange in an input bitmap image. The patent discloses an originalhalftone image, an eroded version (HE), and two dilated versions (HD1and HD2). Then a weight based on descreened versions of the originalhalftone (CO), the color corrected original (CI), the eroded original(CE), and the two dilated originals (CD1 and CD2) is calculated. Thedescreened images are used to select which of the four halftone imagesare transferred into H1 and H2. The weighting function is then used tomerge bitmap versions of H1 and H2 together into the tone scaled outputbitmap (HO). How to descreen is not disclosed, nor exactly how tocalculate which bit of H1 and H2 is used to drive the output bit HO. Theneed to use error diffusion to distribute the error in selecting betweenH1 and H2 is not mentioned. Dilation is described as growing a singlepixel completely around the halftone feature. A second dilation growstwo pixels completely around the halftone feature. Similarly, erosionsubtracts a single pixel completely around the halftone feature. Thispatent fails to teach how to perform descreening.

[0014] U.S. Pat. No. 4,630,125, titled UNSCREENING OF STORED DIGITALHALFTONE IMAGES, inventor Roetling also states that “A partial solutionknown in the art is to spatially filter the halftone image with a lowpass filter.” Roetling teaches that the spatial filter method is not anexact method as it tends to blur the original image.

SUMMARY OF THE INVENTION

[0015] According to the present invention, there is provided a solutionto these problems.

[0016] According to a feature of the present invention, there isprovided a method for converting an original halftone bitmap image by apredefined color correction function to a color converted halftonebitmap image by morphological filtering of the original image tocompensate for tone and color reproduction characteristics comprising:

[0017] providing an original halftone bitmap image;

[0018] providing a set of asymmetrical morphological filters that havethe property that when they are applied recursively in order, thefeatures in the halftone bitmap image are dilated or eroded by varyingamounts by extending or reducing the edges first in one direction, thenin two directions, etc.;

[0019] segmenting the original halftone bitmap image into blocks and foreach block:

[0020] applying said set of morphological filters to said originalhalftone bitmap image to produce a set of modified halftone bitmapimages;

[0021] estimating the percent dot area of the original halftone bitmapimage and the set of modified halftone bitmap images;

[0022] applying a predetermined dot-gain color conversion to the percentdot area of the original halftone bitmap image to produce a modifiedpercent dot area;

[0023] selecting the modified halftone bitmap image whose percent dotarea is closest to the modified percent dot area to produce a block ofthe color corrected halftone bitmap image; and

[0024] replacing said original halftone bitmap image with the combinedblocks of selected modified halftone bitmap image.

ADVANTAGEOUS EFFECT OF THE INVENTION

[0025] The invention has the following advantages.

[0026] 1. The invention provides an efficient technique for adjustingthe dot sizes of bitmap image files such that the color and tone of thebitmap (binary image) can be adjusted prior to printing. These bitmapfiles can consist of single channel bitmaps such as those for a singlecolor device (e.g., a black-and-white laser printer), those for a CMYKoutput device (e.g., a printing press or a graphic-arts proofer), or anyoutput device that utilizes bitmap files of any number of channels(e.g., a multi-ink inkjet printer).

[0027] 2. Another advantage of the present invention is the use of a setof asymmetrical morphological filters results in reduced roundingerrors, when compared with standard erosion and dilation operations,which use larger, symmetrical morphological filters. In the presentinvention, the color and tone of the bitmap files are adjusted byincreasing or decreasing the effective percent dot (image signal) usingmorphological filtering and selection operations.

[0028] 3. Yet another advantage is the error diffusion of the remainingrounding errors between adjacent image blocks reduces the visibility ofthe remaining rounding errors. The technique estimates the local inputimage (dot percentage) level and finds the desired aim from the dot-gaintransformation. The amount that a given dot percentage needs to grow orshrink is defined by a dot-gain curve, which defines the intendedtone-or color-reproduction modification. The features of the inputbitmap image are modified accordingly, using specific morphologicalstructuring elements. The bitmaps, together with the original input formthe candidate bitmap set. Estimates of the resulting local image signalfor each of this set are computed, and compared with the aim signallevel. The bitmap whose signal level is closest to the aim level ischosen, and the associated rounding error is stored. This error issubtracted from the aim signal level for the next local image area(block) in an error diffusion computation.

[0029] 4. Yet another advantage is that the halftone bitmap file can becolor-corrected without having access to the original continuous-toneimage (e.g., scan of photographic film) or artwork. For most traditionaldigital proofing scenarios, this dot-gain compensation is done byre-RIPPING the original art with a modified dot-gain curve. Thismodified dot-gain curve accounts for the dot-gain differences betweenthe target device and the proofing system. Thus, a custom bitmap fileneeds to be generated for each output device. The main drawback to thisparadigm is that the proofs may be made on systems that do not use thesame screening technology as was used to create the target bitmap file.Thus, in order to support a RIP-once-print-many workflow, the dot-gaincompensation needs to be applied to the bitmap file directly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a general flow diagram of the method of the presentinvention.

[0031] Fig. is a flow diagram of the method of local dot percentageestimation and block generation.

[0032]FIG. 3 is a diagrammatic view of example filter coefficients forthe (row and column) filter vectors F_(H) in (365) and F_(Y) in (645)used in the filter-and-downsampling operations of FIG. 2.

[0033]FIG. 4a is a graphical illustration of an example dot-gainfunction showing typical dot-in versus dot-out gain curve.

[0034]FIG. 4b is a graphical illustration of an example dot-gainfunction showing dot-gain percentage as a function of input dot.

[0035]FIG. 5 is a diagrammatic view showing successive extensivemorphological filtering.

[0036]FIG. 6 is a flow diagram showing generation of modified bitmapimages.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention offers an efficient technique to adjust thedot sizes of bitmap image files such that the color and tone of thebitmap (binary image) files can be adjusted prior to printing.

[0038] These bitmap files can consist of single channel bitmaps such asthose for a single color device (e.g., a black-and-white laser printer),those for a CMYK output device (e.g., a printing press or a graphic-artsproofer), or any output device that utilizes bitmap files of any numberof channels (e.g., a multi-ink inkjet printer).

[0039] In the present invention, the color and tone of the bitmap filesare adjusted by increasing or decreasing the effective percent dot(image signal) using morphological filtering and selection operations.

[0040] The main purpose of the present invention is to control the sizeof the dots in a pre-existing halftone bitmap. This bitmap could havebeen generated by a digital screening system (i.e., a RIP) or it couldhave been generated by scanning photographic film to create a digitalbitmap file. In general, these bitmap files were created with someoutput device in mind. As such, the dot pattern was created to produce agiven dot percentage vs. optical density (or lightness, etc.)characteristic that was tailored to a particular output device. If thisbitmap file were printed on an output device that had different dot-gaincharacteristics than the target device, the density as a function ofdot-area percentage will be different. Thus, in order to achieve thedesired color and tone reproduction on a different device, the bitmapfile for the target device must be modified according to the dot-gaindifferences between the target device and the given output device.

[0041] A high-level flow diagram for an embodiment of the method of thepresent invention is shown in FIG. 1. As shown, an original halftonebitmap image file is provided (10). The image is processed in blockssmaller than the total bitmap size (15). Preferably, these blockscorrespond to approximately the halftone cell size of the halftonescreen used to create the bitmap. The dot-area percentage of theseblocks is estimated (15) using a low-pass filter, decimation, andinterpolation process. This process is shown in greater detail in FIG.3. The estimated dot-percentages for the original bitmap (A_(in)) areconverted to aim dot percentages A_(aim) using the dot-gain curve (25)dictating the amount of gain required for the given input dot percentage(20). (An example dot-gain function is shown in FIG. 4). Note: It isunderstood that this function could take on different forms).

[0042] Referring again to FIG. 1, an output bitmap (B) is initialized(30) to be equivalent to the input bitmap (O). (Note: The output bitmaphas the same number of pixels as the input bitmap). The output bitmap Bis adjusted in a sequential manner by processing each (j^(th)) block(35). Thus a conditional loop, controlled by j, is started in (40) toprocess each block in B.

[0043] Several modified bitmap arrays are generated by morphologicallyfiltering B(j) (50). The estimation of the percent dot (mean signal) isdone for each modified array (55) in the same way that was done in (15).The set of signal values A_(D) are compared with A_(aim) in (60) bycomputing a set of difference values. The bitmap whose associateddifference value is the minimum is chosen for storage in B(j) (65). Theminimum difference is taken as the rounding (or selection) error andused in an error diffusion step (75). Here a portion of the error valueis subtracted from the A_(aim) values of neighboring image blocks. Theloop is repeated as long as j is less than J (45). When j=J, thecomplete modified bitmap, B, is stored (200).

[0044] Referring to FIG. 2, the process of estimating the local area dotpercentages of the original bitmap (O) consists of a low-pass filteringand sub-sampling O (15). First a down-sampling factor R is calculatedbased on the ratio of the binary pixel frequency in dots-per-inch dpiand the halftone-screen frequency in lines-per-inch lpi of the bitmap O(600). (This relationship is one example of a process used to specifythe down-sampling rate (R) in practice a different criterion could beused to set this value). The original bitmap is padded to be an integermultiple of the down-sampling rate (605). The process used to create thelocal area dot percentages uses down-by-two decimations followed by afinal bilinear interpolation. Each down-by-two decimation stagecorresponds to a down-sampling rate of 2. Thus there are, log₂(R)down-by-two decimations in R. By definition the number of down-by-twodecimations needs to be an integer. Thus, the number of down-by-twodecimations (n) is calculated by (610):

n=NINT(log₂(R)).  (1)

[0045] Given the padded original bitmap an output continuous tone image(A_(in)) is initialized to be equal to O (615). Next a conditional loopin i, over the range i=1,2,3 . . . n, is established in {625 and 630} toiteratively low-pass filter {635 and 645} and sub-sample {640 and 650}A_(in) n times. During each iteration the low-pass filtering andsub-sampling is performed separably. First A_(in) is convolved with ahorizontal averaging kernel (F_(H)) (635). Next the horizontallyfiltered image is sub-sampled by 2 times in the horizontal direction.This process is repeated in the vertical direction using a verticallyoriented averaging kernel (F_(v)) (645) and a 2 times sub-samplingprocess (650). After A_(in) has been low-pass filtered and 2 timessub-sampled n times it is scaled one final time by the factor 2 ^(n)/Rto its final size (655). (Note: This process illustrated here is onlyone example of a process to estimate the local area dot percentage of O.In the spirit of this invention, it should be recognized that otherprocesses exist for converting a bitmap to a set of local area dotpercentage estimates. As such substitution of one of these otherprocesses should not invalidate the current invention).

[0046] As an example, using a Gaussian-shaped filter for vectors F_(H)in (635) and F_(Y) in (645) leads to reduced artifacts, when compared tothe simple averager of the same length. FIG. 3 lists a length—13convolution filter vector. This is due in part to the fact that atwo-dimensional Gaussian filter is separable into two one-dimensionalfilters (one vertical and one horizontal). This leads to a smoothtwo-dimensional filter spread function and MTF, which in turn reducesthe artifacts in filtered binary images. It should be understood thatthe method of FIG. 3 could be employed with other separable filters, ora non-separable two-dimensional filter, without making the currentinvention invalid.

[0047] Referring now to FIGS. 4a and 4 b there are shown examples of dotgain functions.

[0048]FIG. 4a shows a typical dot-in versus dot-out dot-gain curve, andFIG. 4b shows a curve of dot-gain percentage as a function of input dot.(Note: The same information is present in both curves.)

[0049]FIG. 5 illustrates how an asymmetrical structuring element, s₁,can be used to extend an object in a binary image O. This operation isextensive morphological filtering. An extensive operation is one wherethe set of input data is a subset of the output data. If this operationis repeated successively with other structuring elements, as shown withs₂, the resultant modified bitmap arrays will have varying amounts ofadditional percent dot, when compared to the input original bitmaparray. Applying the same set of structuring elements in anti-extensiveoperations, will results in a set of bitmaps with varying amounts ofreduced percent dot. An anti-extensive operation is one where the set ofoutput data is a subset of the input data.

[0050] Referring to FIG. 6, there is shown in greater detail the process(50) of generating modified bitmaps by morphological filtering. Process(50) includes the following sequential steps: (1) Initialize a set of Pstructuring elements S (500) (2) Initialize a set of 2P + 1 bitmaparrays, D, each of the same size as B (510) (3) Set D(1) = B (515) (4)For p = 2 to P + 1 (520) D(p) = EX[D(p − 1), s(p)] next p (5) D(P + 2) =B*S(P + 2) (525) for p = P + 3 to 2P + 1 D(p) = AE [D(p − 1), S(p)] nextp

[0051] Diffusion of quantization error (ε) (60). In order to reduce thevisual effects of this quantization the error ε can be diffused onto theneighboring pixels of A_(aim) using a suitable error-diffusion scheme(60). One such scheme is error diffusion using the Floyd-Steinbergdiffusion weights. (It should be recognized that this example was chosenfor illustration purposes. In practice other error diffusion schemescould be used.)

[0052] One possible application for the present invention is forRIP-once-print-many systems. These are systems such as digital proofersthat take in press-ready bitmap files and produce adjusted bitmap filesthat can be used in a digital proofing system. In this scenario thepress-ready bitmap file is adjusted such that when printed on a givendigital proofing system the resultant proof has the correct color andtone characteristics.

[0053] The invention has been described in detail with particularreference to certain preferred embodiments thereof, but it will beunderstood that variations and modifications can be effected within thespirit and scope of the invention.

PARTS LIST

[0054]10 read in bitmap file

[0055]15 estimate local dot-percentages in blocks

[0056]20 color correct using dot-gain function

[0057]25 load dot-gain function

[0058]30 initialize output bitmap B

[0059]35 initialize image block loop process and error, ε, to zero

[0060]40 increment block number in loop

[0061]45 test if the maximum number of blocks has been reached

[0062]50 morphological filtering

[0063]55 estimate local dot-percentages for each modified bitmap inblock

[0064]60 signal values compared and selection of modified bitmapw/closest dot-percentage to aim dot percentage. Output error signal, ε

[0065]65 store selected bitmap for block j in output bitmap B

[0066]75 error diffuse ε on to A_(aim)

[0067]200 storage of output bitmap file

[0068]500 initialize a set structuring elements

[0069]510 initialize a set of bitmap arrays

[0070]515 store input bitmap array as a member of initialized bitmaparrays

[0071]520 apply set of extensive morphological filter, recursively andstore results

[0072]525 apply set of anti-extensive morphological filter, recursivelyand store results

[0073]600 calculate down-sampling rate

[0074]605 bitmap padded to an integer multiple blocks

[0075]610 down-by-two decimations calculated

[0076]615 continuous tone image is initialized

[0077]620 set

[0078]625 conditional loop

[0079]630 conditional loop

[0080]635 horizontal averaging process

[0081]640 sub-sample

[0082]645 vertical averaging process

[0083]650 sub-sample

[0084]655 scale dimensions using bilinear interpolation

What is claimed is:
 1. A method for converting an original halftonebitmap image to a color converted halftone bitmap image by morphologicalfiltering of the original image to compensate for tone and colorreproduction characteristics comprising: providing an original halftonebitmap image; providing a set of asymmetrical morphological filters thathave the property that when they are applied recursively in order, thefeatures in the halftone bitmap image are dilated or eroded by varyingamounts by extending or reducing the edges first in one direction, thenin two directions, etc.; segmenting the original halftone bitmap imageinto blocks and for each block: applying said set of morphologicalfilters to said original halftone bitmap image to produce a set ofmodified halftone bitmap images; estimating the percent dot area of theoriginal halftone bitmap image and the set of modified halftone bitmapimages; applying a predetermined dot-gain to the percent dot area of theoriginal halftone bitmap image to produce a modified percent dot area;selecting the modified halftone bitmap image whose percent dot area isclosest to the modified percent dot area to produce a block of the colorcorrected halftone bitmap image; replacing said original halftone bitmapimage with the combined blocks of selected modified halftone bitmapimage.
 2. The method of claim 1 further comprises: calculating thedifference between the percent dot area of the selected modifiedhalftone bitmap image and the modified percent dot area of the originalhalftone bitmap image, to produce an error; diffusing the error toadjacent blocks to further modify the modified percent dot area of theoriginal halftone bitmap image.
 3. The method of claim 1 wherein saidapplying said et of morphological filters includes applying saidmorphological filters to produce a set of dilated and eroded halftonebitmap images.
 4. The method of claim 3 further comprising: testing todetermine whether dilation or erosion is required in order to accomplishthe required dot-gain correction.
 5. The method of claim 3 furthercomprising: applying each morphological filter one at a time, estimatingthe percent dot area of the resulting modified halftone bitmap imageafter each filter; computing the error associated with the selection ofthis modified halftone bitmap image; and determining if the nextmorphological filtering operation is needed in order to minimize theresultant error.
 6. The method of claim 1 wherein said estimating thepercent dot area of a halftone bitmap image further comprises: applyinga linear filter followed by a 2× subsampling operation, repeatedly,until a particular net subsampling of the original halftone bitmap imageis achieved.
 7. The method of claim 1 where the particular subsamplingof the original bitmap image is approximately the size of the halftonecell of the original halftone bitmap image in units of binary pixels ordots.